Electromagnetic horn antenna



March 24, 1959 M. J. EHRLICH 2,879,508

ELECTROMAGNETIC HORN ANTENNA Filed Aug. 2. 1956 Morris J. Ehrlich,

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Patentecl Mal. 24, 1959 2,879,508 ELECTROMAGNETI'C HORN AN'I'ENNA MorrisJ. Ehrlich, Los Angeles, Calif., assignor to Hnglms Aircraft Company,Culver City, Calif., a corporation of Delaware This invention relates toelectromagnetic radiatorsand more particularly to a combination of awaveguide and a sectoral horn.

An H-plane sectoral horn is a horn in which the flaring increases the'aperture in a plane perpendicular to the electric vector. Such hornsoften are employed;to terminate a rectangular electromagnetic waveguidepropagating wave energy in the TE mode to facilitate radiationtherefrom. The radiation pattern from such a horn tenminated waveguidedepends, among other things, on the flare angle and the length of theside walls of the horn; The phase freut of the wave energy radiated bythe H-plane sectoral horn is cylindrical' in a plane perpendicular tothe electric vector and somewhat spherical-in a plane parallel to theelectric vector.

It often has been found desirable to correct or to modify thecylindrical phase wave front in order to obtain a colinear phase front.Even -though the curvature cf the cylindrical phase front may be reducedby increasing the length of the horn, such a solution has beenunsatisfactory. Heretofore modification of the wave freut has beenaccomplished by inserting a dielectric convergent lens of the propencurvature into the aperture of the horn. As it is well known to thoseskilled in the art, a dielectric lens varies in thickness over the planeof its surfaceand thereby produces the necessary phase retardation ofwave energy passing therethrough;to modify thephasje front in apredetermined manner. Such a dielectric lens, however, has thedisadvantage of giving rise to reflections by virtue of the,discontinuity of the interface presented to the wave energy passingtherethrough. Furthermore, the transmission of wave energy through thelens generates heat inside the lens, raising problems of cooling andcausing changes in the transmission characteristics.

It is therefore an object of this invention to provide an H-planesectoral horn afiixed to a waveguide, which combination is capable ofradiating wave energy having a substantially planar phase front withoutthe use of a dielectric lens.

lt is a further object of this invention to provide a waveguideterminated with a short H-plane sectoral horn capable of radiating waveenergy having a substantially colinear phase front.

lt is a still fnrther object of this invention to provide a colinearphase front frorn an H-plane sectoral horn which is rugged inconstruction and which provides an impedance match with free spacesuperior to that achieved with dielectric lenses.

In accordance with this invention, an H-plane sectoral horn terminatedwaveguide is adapted to radiate wave energy with a substantiallycolinear wave front. A sectoral horn is aflixed to a waveguide and waveenergy trapping means are provided across the throat of the combination.Wave energy propagated by the waveguide in the dominant TE mode towardsthe throat of the horn is transformed by the trapping means into a TMsurface wave before reaching the throat, and passed across the throat asa surface wave. Thereafter, the trapping means re-transforms the surfacewave in the dominant TE; mode before reaching the aperture of the horn.In this manner, the efiect of the flaring side Walls is minimized andthe phase front of the wave energy radiated remains colinear.-

The novel features which are believed to be characteristic of theinvention, both as to its organization and method of operation, togetherwith fnrther objects and advantages thereof, will be better understoodfrom the following description conSidered in connection with theaccompanying drawing in which an embodiment of the invention isillustrated by way of example. It is to be expressly understood,however, that the drawing is for the purpose of illustration anddescription only, and is not intended as a definition of the limits ofthe invention.

Fig. 1 is a perspective view of an embodiment of a corrugated wall hornantenna provided in accordance with this invention; and

Fig.2 is a cross-sectional view taken along line 2-2 of,the antenna ofFig.-l.

v Referring n ow to the drawing andparticularly to Fig. 1, there isshowu a rectangular waveguide 10 and an H-plane sectoral horn 12. Thehorn has an aperture 14 and is aflixedto the waveguide 10 at its throat16. The waveguide Wall 18 and the horn wall 20 are provided with aplurality of corrugations 22 which are perpendicular to the direction ofpropagation of the wave energy. The length of the horn, usually measuredas the length of the flared horn Wall 24, is substantially longer thanthe working wavelength of the wave energy propagated through therectangular waveguide 10.

Fig. 2 shows with greater particularity the dimension and osition of thecorrugations 22. The corrugations 22 have their maximum depth at thethroat 16 which depth may be up to one-quarter of the working wavelengthof the wave energy propagated. 011 either side of the throat, the depthof the corrugations is incrementally reduced or tapered so that thedepth decreases slowly to zero. The degree of taper required determinegthe length of the corrugated portion and is usually not critical. Thelength of the corrugatedportion inthe horn d'epends on the length of thehorn and the flare angle of the side walls. It is usually good practiceto c0ntinue the corrugatibns well towards the aperture 14 of horn 12 toobtaina good colinear phase front. Also, the length of the taperedcorrugated portion extending into the waveguide 10 determines the degreeof conversion of the wave energy from the TE mode to the TM mode. Thedistance between adjacent corrugations should be small relative to theworking wavelength. This may be accomplished, for example, by providingat least five corrugations per werking wavelength.

The operation of this embodiment of the present invention may best beexplained by tracing the diiferent modes of transmission of energy fromone and of the waveguide 10 to the horn aperture 14. The waveguidepropagates the wave energy in the dominant or TE mode to the pointwithin the waveguide where the corrugations begin. Upon encountering thetapered corrugations, the dominant mode is gradually converted into theTM mode, also called a trapped surface wave. Thereafter the trappedsurface wave is passed through the throat 16 by the corrugations. Uponencountering the tapered corrugations in the horn 12, the decrease ofdepth of the corrugations causes a gradual leaking ofl. of the waveenergy. The wave energy so released from the trapping snrface againassumes the dominant TE mode. In this manner the eifect of the flaredportion following the throat 16 of the horn 12 is almost completelyeliminated and radiation from the aperture has been found to have asubstantially colinear wave front in a plane normal to the electricvector of the dominant waveguide mode,

Even though the trapping agent employed in this invention has been shownand described in terrns of parallel corrugations, such corrugations arebut an exarnple cf one preferred embodiment. As is well known to thoseskilled in the art, a tapered dielectric slab such as polystyrene mayalso be employed to convert wave energy from the dominant waveguida modeto the surface wave. A dielectric slab of constant thickness attached toametal- -lic surface provides a good surface wave transmisison line.

In the case of dielectric trapping agents, a tapered ortion -of thedielectric material is bonded to the antenna surface perpendicular totl1e electric vector of the dominant mode over the junction section at.the throat. Even though corrugated trapping agents have the advantage ofsuperior ruggedness, dielectric trapping agents are able to capture andsupport surface waves which are cross-polarized. As a consequence, adielectric trapping agent may be bonded to any wall of a waveguide horncornbination to facilitate transfer of wave energy across a horn throatand prevent distortion or curving cf the hase front.

There has been described an improved transition between anelectromagnetic horn and a feed waveguide which provides a substantiallycolinear phase freut at the mouth cf the horn.

What is claimed is:

l. An electromagnetic horn antenna comprising, a waveguide whose axisdefines an antenna axis, an H-plane sectoral horn having a throatcoupled to one end of said waveguide, said waveguide and said horn eachincluding a corresponding broad wall having trapping means adapted totransform the dominant waveguide mode traveling toward said throat intoa surface wave and to transform said surface wave traveling away fromsaid throat into the dominant mode of the mouth of said horn, said trapping means terminating short of the mouth of said horn.

2. An electromagnetic horn antenna comprising, a waveguide whose axisdefines an antenna axis, an H-plane sectoral horn having a throatcoupled to one and of said waveguide, said horn and said waveguide eachhaving a corresponding broad wall including a trapping means, saidtrapping means being disposed Within said waveguide and said horn andadapted to trap wave energy as its thickness increases and to releasewave energy as its-thicknass decreases.

- 3. An electromagnetic horn antenna comprising, a Waveguide including abroad wall whose axis defineg an antenna axis, an H-plane sectoral hornhaving a thront coupled to one end of said waveguide and including abroad wall, each of said broadwalls being provided with parallelcorrugations Wh0se direction of elongation is perpendicular to the axisof said antenna, said corrugations having a maximurn depth at the throatof die horn and tapering to zero depth along either direction parallelto said antenna axis.

. 4. A wave energy horn antenna adapted to provide a wave energy beamhaving a substantially colinear phase front and comprising, a waveguideincluding a first trapping mernber, said first trapping member adaptedto couvert wave energy propagated inthe dominant mode by said Waveguideinto a trappad surface wave, and a wave energy horn coupled to saidwaveguide and including a second trapping mernber adapted to receive andto reconvert said surface wave into the dominant mode cf said horn, saidsecond trapping member terminating short of the mouth of said horn.

5. A wave energy horn antenna adapted to provide a waVe energy beamhaving a substantially colinear phase front and comprising: a waveguidein :luding a first trapping member; and a wave energy horn coupled tosaid waveguide and including a second trapping member, said first andsecond trapping members being arranged to mode transform wave energyinto a trapped surface wave across the junction formed by said waveguideand said horn, said second trapping member being disposed adja- Centsaid first trapping member and arranged within said horn to reconvertsaid surface wava at the throat of tl1e horn into a dominant modeat theaperture cf the horn.

6. A wave energy horn antenna adapted to provide a wave energy beamhaving a substantially colinear Wave 'front and c0rnprising, a waveguidehaving 0ne broad wall including transverse corrugations, and a waveenergy horn coupled to said waveguide and having one broad walliricluding transverse corrugations, said corrugations having a maximumdepth across a junction formed by said waveguide and said horn, thedepth of said corrugations beingtapered to zero depth in the directionnormal to the corrugations for mode transforming wave energy into atrapped surface wave across said junction.

References Cited in the file of this patent UNITED STATES PATENTS 2783467 Gutton et al. Feb. 26, 1957

